This application claims the priority of German Patent Document 198 42 625.9, filed Sept. 17, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for operating an internal combustion engine system as well as an internal combustion engine system operable by such a method. Systems of this kind are used especially in motor vehicles and contain an exhaust purification component in which sulfur in the fuel is contained during operation. Such sulfur-rich exhaust purification components can be, in particular, nitrogen oxide (NOx) storage catalytic converters or so-called sulfur traps.
The sulfur-rich exhaust purification component requires desulfurization from time to time in order to free it of accumulated sulfur, usually in the form of sulfate. Thus, for example, it is known that sulfur poisoning of NOx storage catalytic converters reduces their storage capacity. It is also known that desulfurization takes place preferably with elevated exhaust temperatures and rich exhaust compositions.
Conventionally, desulfurization processes with the engine running are always conducted when the sulfur content in the sulfur-rich exhaust purification component has exceeded a certain amount. This is assumed in the case of a NOx storage catalytic converter, for example, when its storage capacity declines significantly. In methods of this kind, as described in Offenlegungsschrift EP 0 636 770 A1 and German Patent Application No. 197 47 222.2, the declining NOx storage capacity is detected when the adsorption and desorption phases grow shorter. The duration of the adsorption phases can be monitored by a NOx sensor positioned downstream from the NOx storage catalytic converter, while the duration of the desorption phases can be monitored by an oxygen sensor at the same location.
To perform the desulfurization phases, it is proposed in EP 0 636 770 A1 to switch the engine from a lean air ratio to a rich air ratio (in other words, the air/fuel ratio of the air/fuel mixture supplied to the engine) and, if necessary, also to activate an electrical heating unit for the NOx storage catalytic converter. The respective desulfurization phase is maintained for a specified time interval of 10 minutes, for example. In the method in German Patent Application No. 197 47 222.2, the setting of a sufficiently rich engine air ratio is accompanied by addition of secondary air to the exhaust line upstream of the NOx storage catalytic converter. Regulation, and not simply control, of the catalytic converter air ratio (the air/fuel ratio of the exhaust flowing through the NOx storage catalytic converter) can be provided and the catalytic converter temperature can be set to a desired value.
Offenlegungsschrift DE 195 22 165 A1 discloses another method with periodic desulfurization of a NOx storage catalytic converter during engine operation following determination of a decline in its storage capacity. In order to activate a desulfurization phase, (1) a switch is made to a richer engine air ratio and a later ignition point for the respective engine cylinder, and (2) secondary air is also supplied to the exhaust line upstream of the NOx storage catalytic converter. This is preferably performed in such manner that the catalytic converter temperature is set to a desired elevated setpoint during the desulfurization which is maintained for a period of time that can be specified.
The object of the present invention is to provide a method and an internal combustion engine system in which an excessive accumulation of sulfur in a sulfur-rich exhaust purification component is avoided by suitable desulfurization processes that affect normal engine operation as little as possible and do not cause any significant increase in fuel consumption. The internal combustion engine system may be used in automobiles, for example.
This object is achieved by an operating method as well as an internal combustion engine system according to the present invention.
In accordance with a method according to the present invention, a desulfurization process is triggered at each cold start of the engine system to a corresponding desulfurization mode. During the time following a cold-start activation, the engine is usually not operated primarily in accordance with fuel consumption minimization criteria (like those applied for normal operating modes when the engine is warm) because, for example, an attempt is first made in a catalytic converter heating mode to bring the available exhaust purification components, especially one or more exhaust catalytic converters, up to operating temperature as quickly as possible. For this purpose, for example, the engine cannot yet be driven using so-called consumption-favorable stratified charge operation, and appropriate catalytic converter heating measures are advantageous even in engines with direct injection.
Because the engine catalytic converter heating measures, which include, for example, the setting of a rich engine air ratio, largely correspond to the engine measures for desulfurization of the sulfur-rich exhaust purification components, the process according to the present invention does not result in significantly higher fuel consumption by comparison with system operation without desulfurization processes. Since the time intervals after which the next desulfurization process is necessary are typically much longer than the time intervals between successive cold starts, the cold-start desulfurization phases generally suffice to achieve timely and adequate desulfurization without additional desulfurization processes being necessary with a warm engine. As a result, normal engine operation is not disturbed and there is no associated increase in fuel consumption.
In another method according to the present invention, following the activation of an engine cold start, the operation of the engine system is initially set to a catalytic converter heating mode until the temperature of the sulfur-rich exhaust purification components exceeds a minimum desulfurization that can be specified in advance, whereupon operation is switched to the desulfurization mode. The initial catalytic converter heating mode permits very rapid attainment of a sufficient desulfurization temperature for the exhaust purification components to be desulfurized. In another embodiment, secondary air is fed into the sulfur-rich exhaust purification component or into the exhaust line upstream thereof in the catalytic converter heating mode, so that the exhaust temperature is allowed to rise rapidly in conjunction with the selection of the rich engine air ratio. Following a switch to the desulfurization mode, the secondary air feed is terminated.
Another embodiment is suitable for internal combustion engine systems that have an oxidation catalytic converter unit (i.e., an oxidizing function) in the exhaust line downstream of the sulfur-rich exhaust purification component, for example, a 3-way catalytic converter or a NOx storage catalytic converter. According to this method, secondary air is fed into the exhaust line for the oxidation catalytic converter unit during desulfurization, in other words directly into the unit or into the exhaust line section between the unit and the exhaust purification component which is then both desorbing and sulfurrich. This feeding of secondary air permits oxidation of both carbon monoxide and unburned hydrocarbons as well as any hydrogen sulfide produced during sulfurization.
An operating method according to another embodiment is suitable for internal combustion engine systems with two or more sulfur-rich exhaust purification units connected in series. According to this method, the sulfur-rich exhaust purification units in the desulfurization mode are desulfurized in succession, in a sequence which corresponds to the exhaust flow direction. This desulfurization process is accompanied by secondary air being introduced into the exhaust line in each case only downstream of the respective sulfur-rich exhaust purification unit that is being desulfurized. Thus, an undesired secondary air supply to the exhaust purification unit which is currently being desulfurized is avoided and oxidation of carbon monoxide, unburned hydrocarbons, and any hydrogen sulfide that may result during desulfurization is ensured.
In a method according to another embodiment of the present invention, which includes the catalytic converter heating mode followed by the desulfurization mode following cold-start activation, the engine air ratio is advantageously set to be slightly rich in the desulfurization mode, in other words richer in fuel than the stoichiometric ratio, but poorer in fuel than in the catalytic converter heating mode, which has a favorable effect on fuel consumption.
According to another method of the present invention, the duration of the respective desulfurization mode is determined by using (1) a sensor to monitor the sulfur storage state of the sulfur-rich exhaust purification component, or (2) a model-based estimate. In such an estimate, in addition to the quantity of fuel consumed and the sulfur content of the fuel, natural desulfurization processes that occur from time to time are also taken into account. These include desulfurization processes that occur when the engine has been warmed up when, because of the current engine operating state, desulfurization-promoting conditions prevail in the sulfur-rich exhaust purification component, especially a sufficiently high temperature and a sufficiently rich air/fuel ratio of the exhaust, for example, during highway and/or full-load driving.
The internal combustion engine system according to the present invention includes at least two sulfur-rich exhaust purification units connected in series in the exhaust line, as well as secondary air supply means each of which contains a separate secondary air supply branch for the sulfur-rich exhaust purification units. In this way, a deliberate secondary air supply for the respective sulfur-rich exhaust purification components is possible, for example, to bring these components more rapidly to operating temperature or to oxidize hydrocarbons as well as carbon monoxide and/or hydrogen sulfide contained in the supplied exhaust.
The internal combustion engine system may also include an oxidation catalytic converter unit downstream of the sulfur-rich exhaust purification component, which can comprise one or more exhaust purification units in series. The secondary air supply means includes, in addition to one or more secondary air supply branches for the sulfur-rich exhaust purification components, an individual secondary air supply branch for the oxidation catalytic converter unit, so that, for example, hydrogen sulfide can be oxidized in this unit that is formed during a desulfurization process in the upstream sulfur-rich exhaust purification component.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.